1. Field of the Invention
This invention relates to a multimedia information receiving apparatus for receiving multimedia information typically prepared by multiplexing an audio signal and a moving image signal that are related to each other and compressed separately, demultiplexing and expanding the audio signal and the moving image signal and reproducing them synchronously. More particularly, the present invention relates to a moving image packet decoding and reproducing apparatus and also to a computer program product for controlling the reproduction time adapted to be used with such an apparatus.
2. Description of the Related Art
Generally, in digital broadcasting, a transmission method of digitally coding and compressing a moving image signal (V) and an audio signal (A) and subsequently multiplexing the signals to produce multimedia information and transmit it in a single stream is used. In a system using such a transmission method, the two signals including the moving image signal and the audio signal have to be synchronously reproduced at the signal receiving side. In view of this problem, MPEG-2 (Moving Pictures Experts Group) provides that the operation of synchronously reproducing the moving image signal and the audio signal is controlled by using a piece of output timing information that is referred to as time stamp.
An AV synchronizing method that is proposed for a transmission system using an MPEG-2 transport stream (MPEG-2TS) and currently popularly being used for digital broadcasting will be described below.
The moving image signal and the audio signal are divided into units of decoding and reproduction (e.g., a frame in the case of a moving image) that are referred to as access units, which are then transformed into packets referred to as PES (Packetized Elementary Stream) in the apparatus comprising an encoder of the transmitting side. At this time, a piece of time control information (Time Stamp) for signal reproduction and output that is referred to as PTS (Presentation Time Stamp) is added to the header of each packet of the moving image signal and the audio signal. The decoder of the receiving side contains a timer called STC (System Time Clock) and each of the access units is reproduced and output when the value of the timer (reference time) agrees with the PTS of the corresponding packet.
In real time transmission, it is necessary for both the transmitting side and the receiving side to synchronize the signals on a real time basis in a controlled manner, using a buffer control technique, in order to consummate the transmission/reception successfully.
With MPEG-2TS, each PES packet is further divided into smaller packets referred to as transport stream packets (TSs) having a fixed length. At this time, the transmitting side adds a signal called PCR (Program Clock Reference) to the header of the TS packet for the purpose of calibration of reference time and the receiving side constantly monitors the PCR and the STC and, if necessary, correct STC in order to establish synchronism of signal transmission and signal reception.
In recent years, systems for streaming media designed to deliver multimedia information by way of the Internet and multimedia data broadcasting using a narrow band have been started. These systems mostly employ a moving image coding method conforming to MPEG-4, an audio coding method typically conforming to AAC (Advanced Audio Coding) or AMR (Adaptive Multi rate) and an A/V synchronizing method using a time stamp similar to the one provided in MPEG-2.
Meanwhile, in conventional digital broadcasting systems where the transmission bit rate is as high as several Mbps, it is possible to correct errors in time stamp information firmly and reliably. In other words, it is not conceivable that errors are contained in time stamp information. In the case of delivering multimedia information by way of the Internet, it is not conceivable that errors are contained in time stamp information because of the transmission system is adapted to use a TCP (Transmission Control Protocol).
However, in the case of data transmission systems and mobile telephone systems comprising mobile communication terminals where the transmission bit rate is low, it is not allowable to assign a wide band to the feature of correcting errors in the system data area including time stamps. Furthermore, when multimedia contents are transmitted on a real time basis by radio transmission, measures not involving retransmission will more often than not be taken as in the case of UDP (User Datagram Protocol), although errors due to radio transmission may additionally be involved. Then, it is probable that errors frequently occur in time stamps. Particularly, since the volume of data of a moving image is large, errors can occur in the time stamps in the moving image signal. Then, the moving image can be displayed at irrelevant timing to miss the synchronism with the reproduction and output of the corresponding sound.